1. Field of the Invention
The invention relates to steam turbine bell seals for sealing the main inlet pipe which introduces steam to the nozzle chamber through the turbine casings.
2. Description of the Prior Art
In steam turbines, it is necessary to provide a sealed passage for steam between the separate casings of the unit. This is accomplished either by inlet pipes or snouts surrounded by a series of seal rings, or, as in the case of certain turbines, by a bell seal. Since the casings have different rates and magnitudes of thermal expansion these seals must be capable of accepting vertical, axial and transverse differential expansion relative to the axis of the turbine shaft, without permitting significant leakage.
The main elements of a prior art bell seal assembly are the bell seal, a retainer nut, and a locking screw. In conventional designs, the bell seal is made of stellite material with the nut, locking screw, inlet sleeve and nozzle chamber neck made of CR/MO steel.
In the steam path of the turbine, maintaining efficiency is dependent on the tightness of the seals. The tighter the seals, the more efficient is the turbine. Bell seals are no exception. They must be able to move and accommodate the normal expansion and contraction that takes place as units heat up and cool down. However, when the units are taken apart, in most instances it is found that the bell seals are either cracked, distorted or "frozen" in place, leading to difficulty with disassembly, excess clearances, increased steam leakage, and decreased turbine efficiency due to a high buildup of iron oxide.
As temperatures in excess of 700.degree. F. are reached, a thin layer of oxide buildup forms on all ferritic material surfaces. This layer can sometimes be in excess of 0.010" thick and interferes with the tight radial and axial clearances required in the bell seal assembly. For instance, as the oxide layer forms on the retaining nut, it quickly takes up the axial clearance between the nut and the bell seal, locking the bell seal in place. When differential expansion occurs, the seal cannot move, creating undue stresses on the assembly and distortion and stretching of parts leading to steam leakage. To further complicate matters, if particulate material is contained in the steam coming from the boilers, erosion takes place on steam inlet components resulting in costly repairs. In some cases, the bell seal becomes so locked up that it must be destroyed during disassembly, creating expensive and difficult replacement.
The purpose of a bell seal is to seal the area between the steam inlet pipe and the nozzle area. Steam is flowing through this cavity at a temperature of approximately 1000.degree. F. and at a pressure of approximately 3500 lbs. Psi. This system allows for 0.25 to 0.5 percent leakage between the seal and the inlet pipe. The reason for this leakage is to allow for assembly of the components, that is, there is a 0.002 to 0.003 clearance between the seal and the inlet pipe. More clearance, which would be preferable to improve ease of assembly and freedom of movement in operation, would allow much greater leakage which is detrimental to the turbine efficiency. There is also a 0.003 to 0.004 clearance by design between the nut and the bell seal. This clearance is to allow for radial movement between the nozzle chamber and the inlet pipe that occurs because of the thermal transits during start up of the turbine. The events that occur when the two pipes are at ambient temperature of 68.degree. F. and are suddenly exposed to a temperature approximately 1000.degree. F. and a pressure of approximately 3500 lbs. Psi cause significant thermal transits between the two pipes. The nut used to attach the bell seal to the inlet pipe is made from CR/MO. Although this material has adequate strength, it is highly susceptible to oxide buildup, thus making it impossible or nearly impossible to remove it by unscrewing in the normal fashion. The nut in most cases must be cut out, a very difficult and time consuming process. The bell seal is manufactured from Stellite-6 which is a highly erosion resistant and tough material. The nozzle chamber is made from CR/MO which is a relatively soft and malleable steel and susceptible to oxide buildup. After operating in a harsh environment for several years the seals are very difficult to separate without galling the nozzle box during disassembly. When galling does occur it becomes necessary to machine the nozzle chamber, a very time consuming and expensive operation.
To summarize the problems described above:
small assembled clearances, which makes disassembly/reassembly difficult and restricts differential movement between major turbine components; PA0 high oxidation rates which promote binding between the seal area; PA0 high oxidation rates which promote binding between the nut and the inlet pipe not allowing the parts to move relative to each other; PA0 high coefficients of friction that can damage the tight seal area and restrict movement; PA0 the bell seal, the locking nut, and the nozzle chamber are often destroyed or damaged during disassembly; and PA0 the designed leakage between bell seal and the nozzle chamber.